The Relative Efficiency of Gasoline, Natural Gas, and Propane Fuels For
Back-Up Generators
By Bruce Dishongh

When purchasing a generator as a back-up power supply for your home, one
thing to consider is which type of fuel you will be using. While there are
several other important factors when buying a generator, this article is only
concerned with the method of comparing the relative efficiency between
gasoline, natural gas and propane.

The first thing to understand is the equivalent ratio of energy output
for the three fuels as expressed in BTUs, the commonly used unit of energy. The
following table will demonstrate:

·Gasoline 1 gallon = 125,000 BTUs

·Natural Gas 1 CCF = 100,015 BTUs (CCF=100 cubic feet)

·Propane 1 gallon = 91,700 BTUs

You can see from above that 1 gallon of gasoline is more efficient than
100 cubic feet of natural gas or 1 gallon or propane. In fact, the ratio is
approximately:

·1 gasoline = 1.25 CCF natural gas = 1.36 gallons
propane

In other words, you would need 1.36 gallons of propane to produce the
equivalent BTUs of 1 gallon of gasoline; or, 1.25 CCF of natural gas.

As a practical example let's say that gasoline is currently $2.85 a
gallon, natural gas $.95 per CCF, and propane $4.00 a gallon (these are today's
prices where I live). Next, my generator uses 10 gallons of gasoline a day if
run continuously for 24 hours. Therefore, for one day's usage I need:

As you can see, once you know the relative efficiency of the three fuels
you can just plug in the current prices of the fuels to calculate the daily
cost of running your generator for each fuel. However, in the case of propane,
the cost per gallon can vary widely depending on the number of gallons purchased.
In the example above, $4.00 a gallon was for filling up a small 4-5 gallon
container; for larger purchases the price can decrease substantially.

If you are thinking of buying a generator it is best to consider more
than just the initial purchase price. If you will be using it for extended
periods of time it could be cheaper in the long run to buy a generator capable
of running on alternative fuels.